Electrical motor drive systems having a cyclic load which require energy to be cycled into and out of the load are common in industrial applications. An example of such a system is a cut-to-length mechanism which involves the acceleration of the cutting mechanism from rest up to a speed matching that of a moving web in order to cut the web cleanly, and the subsequent deceleration of the cutting mechanism to rest awaiting the next cut. Such cutting mechanisms are generally massive in size, and the energy transferred into and out of the cutting mechanism during the cyclic accelerations and decelerations of the cutting mechanism is very substantial. Because of this, the total power required by the drive system is determined primarily by the power required to accelerate the cutting mechanism from rest; the power required for the cutting operation being relatively small in comparison.
Drive systems for applications such as the foregoing application typically are designed to regenerate the load energy to the power line during deceleration, utilizing well know techniques such as phase controlled rectifiers operating in an inverting mode. Even though the regeneration of power to the power line does conserve a substantial portion of the energy, it transfers this energy at a high power level and, typically, at a very poor average power factor. Because of this, the peak KVA demand on the power system, including the power lines, transformers and switchgear is very high relative to the average KW requirements. This results in increased costs to the user since the power handling devices are sized to handle the peak KVA requirements. In addition, many electric utility companies impose penalty charges based upon peak KVA demand.
Because of the foregoing, it has become desirable to develop an electric motor drive system wherein the energy generated by a cyclic load is utilized within the system so that the power requirements, as seen by the power lines, reflect only the average net power required by the load.